1. Field of the Invention
The present invention relates to a gas detecting apparatus capable of being used in detection of a state of a gas, for example, concentration measurement or leak detection of a combustible gas present in a detected atmosphere, or detection of the humidity of the detected atmosphere.
2. Description of the Related Art
As a gas detecting apparatus, for example, a heat conductivity humidity sensor is known in the related art. Such a heat conductivity humidity sensor is a sensor for measuring humidity using a change in thermal radiation from a temperature measurement resistor heated by a heating element for self-heating by Joule heat.
As this kind of heat conductivity humidity sensor, a technique for switching and controlling a heating resistor at two temperatures using a Wheatstone bridge circuit and obtaining humidity from a voltage applied to a heater at that time has been disclosed in recent years (see Patent Reference 1).    [Patent Reference 1] JP-A-H09-5284
3. Problems to be Solved by the Invention
However, in the related-art technique described above, an output voltage from both ends of the heating resistor is directly inputted to an arithmetic circuit (microcomputer), so that there is a problem that a resolution of the output voltage (measured voltage) is limited by an input resolution of the arithmetic circuit.
That is, in the heat conductivity humidity sensor using the heating resistor, since it is necessary to detect a very small change in voltage, there is a problem that measurement accuracy becomes low when the resolution of the measured voltage is low.
As a countermeasure, it is contemplated to use an arithmetic circuit having high resolution, but there is generally a problem that the high resolution also increases the price of the arithmetic circuit.
It is contemplated to amplify the measured voltage by an amplifier. However, since it is necessary to set the voltage after amplification at a power supply voltage or less, an amplification factor cannot be increased too much. This results in a problem that high measurement accuracy cannot be obtained.